Arc vs String, is Arc really faster? (blocklisted.github.io)

submitted 11 months ago by snaggen@programming.dev to c/rust@programming.dev

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[-] nous@programming.dev 21 points 11 months ago* (last edited 11 months ago)

TBH, I find the whole premise a bit silly. The one use case the original video talks about there this might be a good idea is when you have something like a MonsterId(String), and argues for MonsterId(Arc) instead - which I think is an anti-pattern. Use MonsterId(usize) and it becomes far cheaper to clone than even an Arc. There is no need to lug around strings as ids - which are presumably going to be used to look up values in a hashmap or similar (which again, I would suspect it is faster to has a usize than a string of any type).

Most of the rest of the time cloning a string is not a huge issue or if it is &str is often good enough. I find it rare that you would ever need the clone performance and multiple ownership of a Arc.

There are also crates like smallstring, smallstr etc, that are stack allocated strings up to a certain limit before they start allocating. Which would be worth a look at and see how they preform compared to Arc or String. Since most things I bet they were using Strings for would fit inside these types just find without the extra allocation.

It is a neat trick to have in your toolbag - but not the first thing I would jump for when dealing with strings.

[-] Ephera@lemmy.ml 2 points 11 months ago

which are presumably going to be used to look up values in a hashmap or similar (which again, I would suspect it is faster to has a usize than a string of any type).

Yeah, for any primitive types, you can just use the value itself as the hash value here. So, effectively a noop. I assume, Rust's implementation makes use of this...

[-] MantisWaffle@lemmy.world 4 points 10 months ago

Probably not for ddos/security reasons. Would need to use something like nohasher to get noops.

[-] Ephera@lemmy.ml 1 points 10 months ago

Hmm, I was wondering, if there's an overlap with using hashes for security stuff. Do you happen to know of an exploit that makes use of something like predictable placement in a hashmap?

Or is your assumption rather that they wouldn't include special treatment for primitive types in the hashmap implementation?
It definitely feels a bit freaky to me, too, since you'd rob users of the ability to customize the Hasher implementation, but I also felt like they almost have to do it, because it might make a massive difference in performance.

...but after thinking about it some more, I guess, you'd typically use a BTreeMap when your keys are primitives. So, now I'm on board with the guess, that they wouldn't include special treatment into HashMap. 🙃

[-] nous@programming.dev 5 points 10 months ago

It is talked about in the hashmap docs:

By default, HashMap uses a hashing algorithm selected to provide resistance against HashDoS attacks. The algorithm is randomly seeded, and a reasonable best-effort is made to generate this seed from a high quality, secure source of randomness provided by the host without blocking the program.

The default hashing algorithm is currently SipHash 1-3, though this is subject to change at any point in the future. While its performance is very competitive for medium sized keys, other hashing algorithms will outperform it for small keys such as integers as well as large keys such as long strings, though those algorithms will typically not protect against attacks such as HashDoS.

Basically, if the attacker has control over the key inserted into a hashmap then with a simple hashing algorithm they can force collisions which results in the hashmap falling back to a much slower linear lookup. This can be enough to stress a server and slow down all requests going through it or even cause it to crash. So a lot of effort is made in the default hasher to mitigate against this. There are faster hashing implementations out there if you are not worried about this that you can opt into. But the default is to be secure.